Wow, you actually have many questions in one here. Unfortunately many of the answers are going to vary by insurance carrier. I will try to answer generically the best I can.
First, on PT & OT being done on the same date. This is definitely going to depend on the insurance you are billing. I'm assuming the PT & OT are both billing under the same group name & tax ID # which will make the services look like duplicates. I would recommend billing them out on separate claims (PT on one, OT on another). You can either wait for one to be denied as a duplicate and appeal it with an explanation, or attach a note of explanation to the claim on the first submission (my recommendation).
As far as the "therapy camp" (which sounds like it could be quite effective and make the therapy fun for the kids) again, it is going to depend on the insurance you are billing. Most insurances have either a global allowance or a set fee they allow per visit so even if you bill 10 units and a modifier, it most likely won't matter. For those companies you may want to contact provider relations and ask if there is any review process so that they can consider the specifics. If you can't contact the provider relations I would submit the claims along with a note of explanation and if they are still processed at a very low allowance, send in a written appeal explaining in detail what therapy was performed.
As for some insurances(unfortunately, not very many), they will just process your claim as submitted with the units etc.
If it is feasible for you I would go ahead and attach the notes, with the note of explanation to the claims on the first submission. It may reduce time in getting the claim appealed if they already have all info on file.
I probably would have called prior to the therapy camp date to inquire (maybe just a couple of the major insurance carriers) to see how they would consider this type of treatment. But hindsight is 20/20.
Hope this is helpful!
Michele